Quantum mechanics is the fundamental physical theory that describes the
behavior of matter and of light; its unusual characteristics typically occur
at and below the scale of atoms. It is the foundation of all quantum
physics , which includes quantum chemistry, quantum field theory, quantum
technology, and quantum information science.
Quantum mechanics can describe many systems that classical physics cannot.
Classical physics can describe many aspects of nature at an ordinary
(macroscopic and (optical) microscopic) scale, but is not sufficient for
describing them at very small submicroscopic (atomic and subatomic) scales.
Classical mechanics can be derived from quantum mechanics as an approximation
that is valid at ordinary scales.